1. Field of the Invention
This invention relates to the recovery of transition metals and more specifically, this invention relates to the recovery and purification of transition metals from nitric acid liquors.
2. Background of the Invention
Transition metals are important adjuncts in a myriad of endeavors, including metallurgical applications, catalysts, and nuclear medicine. For example, molybdenum is used in the radiopharmaceutical industry as a source for 99Mo/99mTc generators. Sources of transition metals include natural resources (ores), fission products, or irradiated medical waste streams.
Mo-99 can be generated by (n,γ) reaction from enriched Mo-98 in a nuclear reactor, or by (γ,n) reaction from enriched Mo-100 using a particle accelerator. Also, Mo-99 can be produced through fission of U-235.
Several methods exist to recover metals from low concentration feeds, including ion exchange and solvent extraction. However, given that mining and metal recovery is associated with highly variable feed solutions, the impurities associated with some feed streams can foul ion exchange resins.
Current methods for harvesting transition elements from ores include the use of sulfuric acid and hydrochloric acid and sulfate solutions to generate pregnant leach solutions. Both sulfuric and hydrochloric acids are very aggressive media that may dissolve stainless steel equipment. Therefore, extractions are typically conducted at positive pH values to minimize the corrosion. However, these positive pH extraction protocols often confound extraction attempts from liquors having negative pH values.
These methods also utilize very alkaline inorganic stripping reagents such as ammonium hydroxide and/or sodium hydroxide at pHs ranging from about 5 to 11. These reagents are extremely corrosive and therefore dangerous to personnel, and damaging to equipment. There are also environmental risks associated with the generation of secondary waste streams containing such caustic fluids.
Other protocols require complex extraction liquors. For example, one or more organic complexing agents are often required to be combined with a plurality of extractants to effect harvesting of targeted metals. These combined solvent systems are complex to implement and also costly. Also, these combined solvent systems co-extract different metals or classes of metals simultaneously, such as Lanthanides and Actinides. Lastly, many of these combined solvent systems are utilized to treat nuclear fuel, which contains unusable forms transition metals.
High temperature roasting is another way to harvest certain transition metals, such as molybdenum. High temperature (approximately 500° C. to 600° C.) furnaces can remove sulfur from sulfuric acid/Molybdenum liquors and change the chemical form of molybdenum. But such high temperature operations create additional risks, and also are expensive.
A need exists in the art for a method for isolating transition metals present in a myriad of concentrations (e.g., from about 1 pM to about 600 mM) for a variety of feed streams. The method should be environmentally friendly and safe to personnel. Furthermore, the method should also utilize an aqueous acid phase and an organic phase wherein the acid confers resistance to corrosion of extraction hardware. The method should supply relatively pure metal (above 95 percent) in a final aqueous phase.